upudate minimal example to ensure bugfree for entry user

closes #3

README.md

@@ -23,10 +23,11 @@ pip install git+https://github.com/ACEsuit/mace.git
 
 ```python
 from mace.calculators import MACECalculator
-from ase import Atoms
+from ase import Atoms, units
+from ase.build import bulk
 from ase.md.npt import NPT
 
-atoms = Atoms("NaCl")
+atoms = bulk("NaCl", crystalstructure='rocksalt', a=3.54, cubic=True)
 
 calculator = MACECalculator(
   model_paths=/path/to/pretrained.model,
@@ -36,14 +37,16 @@ calculator = MACECalculator(
 
 atoms.calc = calculator
 
-npt = NPT(
+dyn = NPT(
   atoms=atoms,
   timestep=timestep,
   temperature_K=temperature,
   externalstress=externalstress,
+  ttime=ttime,
+  pfactor=pfactor,
 )
 
-npt.run(steps)
+dyn.run(steps)
 ```
 
 # Citing